hapten

a substance having a single antigenic determinant that can react with a previously existing antibody but cannot stimulate more antibody production unless combined with other molecules; a partial antigen.

hapten hap·ten (hāp'tən)n. A substance that is capable of reacting with a specific antibody but cannot induce the formation of antibodies unless bound to a carrier protein or other molecule. Also called incomplete antigen, partial antigen.